In a harvester-thresher, there is still some chaff and short straw mixed with the threshed grain, as a rule, after the threshing and separation. The cleaning device removes these impurities from the grain. With most harvester-threshers, the cleaning device contains three main components: a blower, an upper screen, and a lower screen. The blower has its own housing, whereas the upper screen and the lower screen are located on a screen box.
An adjustable screen is constructed from a number of metal sheets, extending transversely, with rows of teeth. Each of these metal sheets is affixed to a crankshaft, which has a crank arm, which is found engaged with an adjustment rod extending axially. By means of an axial displacement of the adjustment rod, all metal sheets are simultaneously moved—that is, swiveled around their longitudinal axis. The screens are acted on with an air flow by the blower from below, which air flow, jointly with a shaking movement of the screen box, leads to a situation where the lighter impurities are carried away to the back by the air flow, whereas the heavier grain falls between the metal sheets of the screens and the grain passing through the lower screen is conveyed into the grain tank. The mixture of grain and chaff, yielded at the end of the lower screen, is conducted, as tailings, once again, to a threshing process, whereas the mixture yielded on the end of the upper screen is discharged onto the field.
The metal sheets of adjustable screens usually have an upper half, equipped with teeth, which extends from the rotating axis at an incline toward the back and upward, and a half extending from the rotating axis downward. In the state of the art, it is common to shape all metal sheets of a screen in a similar manner. Accordingly, they all have the same shape, length, and width, number of teeth, etc.
Since in many harvester-threshers, a conveying tray, which extends at an incline toward the front and downward, follows on the end of the screen downstream and is used in the case of the lower screen to convey the grain to an auger conveyor for grain, which conducts it to an elevator, which conveys it to the grain tank and which is used to convey the tailings to a tailings auger conveyor in the case of the upper screen, which returns incompletely threshed ear parts back to the threshing process through another elevator, it is possible for an accumulation of air to arise in such cleaning devices at the back end of the screen, which leads there to a higher pressure and a higher air speed of the air flow passing between the metal sheets than in the front area of the screen. The separation of the grain from the chaff and the short straw fractions takes place, accordingly, essentially in the back area of the screen, whose front area on the other hand contributes relatively little to the total throughput performance of the screen. In such cases, the theoretically possible throughput of the cleaning is not attained.
U.S. Pat. No. 3,043,427 A and DE 1 607 633 A propose the placement of several conducting crosslinks, rising transversely and in the direction of flow, against the screen underside below the screens. In this way, partial air flows with the most favorable intensities will be conducted to the individual longitudinal areas of the screen. These conducting crosslinks are affixed rigidly and the screen metal sheets cannot be adjusted. If one were to use such conducting crosslinks on a screen with adjustable metal sheets as proposed in DE 43 25 310 A, that would result in changing the flow ratios on the conducting crosslinks with an adjustment of the screen metal sheets since, with a larger screen opening, more air already flows through in front of the conducting crosslinks upward between the screen metal sheets than with a smaller screen opening. The conducting crosslinks thus would have only a relatively small effect, if the metal sheets are opened, whereas their effect is relatively large with metal sheets that are further closed. An optimal air distribution can, accordingly, be attained only for a certain, prespecified opening of the screen metal sheets, so that a reasonable adaptation possibility of the screen opening to various types of crops or crop characteristics, such as moisture or throughput, is not available
U.S. Pat. No. 3,374,886 A proposes the placement of baffle plates, which can be adjusted below V-shaped screen elements and which deflect the wind upward. Such baffle plates are distributed over the entire length of the screen and are jointly adjusted in their inclination using a single lever. The screen elements themselves do not have adjustable metal sheets. If grain of different grain sizes is to be processed, then a change of the screen proves to be necessary.